1. Field of the Invention
This invention relates to 4-halopyridines. In particular, this invention relates to a process for stabilizing 4-halopyridines.
2. Description of the Art
4-Halopyridines are compounds which are unstable in the course of storage at room temperature and are obtainable synthetically via methods known per se, for example by chlorinating the corresponding 4-hydroxypyridine with PCl5 or POCl3 (Wibaut & Broekmann, Recueil des Travaux Clinique des Pays-Bas et de la Belgique 7659, 78, 593-603). Impurities from the preparation process, especially residues of acid, lead to the decomposition of the 4-halopyridines and the formation of oligomeric products. One product formed in the case of 4-chloropyridine is the salt of the N-(4′-pyridyl)-4-chloropyridinium chloride dimer (Wibaut & Broeckmann, Recueil des Travaux Clinique des Pays-Bas et de la Belgique 1961, 80, 309-312).
Den Hertog et al. (Wibaut & Brockmann, Recueil des Travaux Clinique des Pays-Bas et de la Belgique 1951, 70, 105-111) state that 4-nitropyridine can be stabilized by the addition of 10% aqueous sodium bicarbonate solution, in which case 90% of 4-nitropyridine were still present unchanged at 60° C. after 12 hours.
According to Wibaut & Brockmann (Recueil des Travaux Clinique des Pays-Bas et de la Belgique 1961, 80, 309-312), 4-chloropyridine when mixed with primary amines NH2R (R═CH3, CH(CH3)2, C12H25) or secondary amines NHR′2 (R′═CH3, n-C4H9), the formation of the corresponding (4-NHR)- and (4-NR′2)-pyridines respectively is observed, while aliphatic tertiary or mixed aliphatic-aromatic tertiary amines (NEt3, diethylaniline) did not react with 4-chloropyridine. 4-Chloropyridine forms an N-(4′-pyridyl)pyridinium salt with the aromatic tertiary amine pyridine. 4-Chloropyridine is currently commercially available only as the hydrochloride, in which the changed electronic conditions in the form of pyridinium salt stabilize the molecule.